Focussing reflector with dimpled surface to scatter infra-red radiation



Feb. u, my 1 WI M. 3,427,625 Pocussme LE R TH MPLED SURFACE T0 50 ER F FRED RADIATION Filed Dec. 14, 1962 Sheet of 2 Fig. 5.

IN VEN TOR Mahmoud I. Kazimi BY Feb. 11, 1969 M. 1. KAZIMI 3,427,625

F'QCUSSING REFLECTOR WITH DIMPLED SURFACE TO SCATTER INFRA'RED RADIATION Sheet Filed Dec. 14 1962 Fig.4.

Fig. 5.-

INVENTOR. Mahmoud I. Kazimi BY United States Patent 3,427,625 FOCUSSING REFLECTOR WITH DIMPLED SURFACE T0 SCATTER INFRA-RED RADIATION Mahmoud I. Kazimi, Berkeley, Calif., assignor to Hexcel Corporation, Berkeley, Calif., a corporation of California Filed Dec. 14, 1962, Ser. No. 244,728 U.S. Cl. 343-840 Int. Cl. H01q 19/14 The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

This invention relates to a focusing reflector for radiated electromagnetic energy and more particularly to such reflector that discriminates against interfering electromagnetic energy of shorter wavelength than the wavelength of the radiated energy.

The preferred embodiment of the invention described herein and shown in the drawings comprises a core formed of cellular honeycomb constructed of plural bonded ribbons, the edges of which ribbons are shaped to define the contour of a concave surface such as a paraboloid. The honeycomb material has a cell size no greater than one quarter wave length of the energy being reflected so that the surface of the honeycomb material will be substantially opaque to such wavelength and will reflect the same. Signals having shorter wavelengths will pass through the individual honeycomb cells and thus will not reflect to an antenna spaced relative to the concave surface.

Operation of paraboloid reflectors such as parabolic reflectors in an environment in which infra-red and shorter wavelength radiation is present requires special precaution to prevent excessive heating of the reflector and to assure that the intensity of shorter wavelength radiation does not exceed the intensity of the lower frequency intelligence containing signals. The foregoing considerations have become particularly important in relation to the placement of antennas in space vehicles, because the intensity of infra-red and shorter wavelength radiation is greater in outer space since the radiation is not impeded by the ionosphere. Reflectors having the foregoing electrical characteristics must also have sufficient rigidity that the reflecting surface thereon maintains a desired shape. Additionally, an antenna of the type here under consideration should be lightweight, simple, and inexpensive to produce.

An object of this invention is to fiulfill the foregoing desiderata.

Another object is to provide an antenna that has structural rigidity and has a reflective surface with electrical 3 Claims continuity and frequency selectivity so that higher interfering frequencies will not be reflected by the device.

Still another object is to provide a method for expeditiously forming an antenna with the last named characteristics. The object includes providing a piece of honeycomb with a suitable contoured concave surface thereon and providing a thin sheet of conductive material, aflixing the skin to the concave honeycomb surface, and creating a pressure differential across the thin sheet so that the thin sheet will be partially drawn down into the honeycomb cells. A myriad of small concavities or dimples will not have substantial effect on relatively longer wavelength signals but will have substantial effect on shorter wavelength signals. The latter will be focused at a point very near to the surface of the skin and will there be dissipated, rather than being reflected to an antenna disposed in front of the concave surface to receive or transmit the longer wavelength signal.

Alternate embodiments of the invention can be constructed in which a thin sheet of conductive material is aflixed to the curved honeycomb core and is provided with a plurality of holes spaced from one another by an amount no greater than one quarter wave length of the signal being reflected. The thin layer can also be formed of conductive mesh attached to the edges of the honeycomb forming ribbons. These two modifications provide a path through the anntenna for dissipation of thermal energy contained in infra-red and shorter interfering frequencies. Energy that passes through the openings in the conductive face will with high probability also pass through the honeycomb cells. Thus, the reflector is not heated to a temperature high enough to cause destruction thereof by the interfering short length waves.

These and other objects will be more apparent after referring to the following specification and attached drawings in which FIG. 1 is a cross-sectional elevation view of a honeycomb core forming a parabolic reflector surface;

FIG. 2 is an enlarged view of a portion of the honeycomb core taken substantially along line 2-2 of FIG. 1;

FIG. 3 is an enlarged cross-sectional elevational view showing another embodiment of the present invention;

FIG. 4 is an enlarged view of a portion of the antenna illustrating still another embodiment of the invention;

FIG. 5 is a cross section view view taken substantially along line 55 of FIG. 4;

FIG. 6 is a plan view of yet another embodiment of of the present invention; and

FIG. 7 is a cross section side view of the embodiment of FIG. 6.

Referring more particularly to the drawings, reference numeral 12 denotes generally a honeycomb core formed by a plurality of flat ribbons 14 bonded at recurring intervals as at 16 to form a plurality of cells 17. The type honeycomb shown in FIG. 2 is hexagonal honeycomb but it should be understood that any suitable shape cell structure can be provided within the scope of the present invention. Core 12 is preferably formed with cells wherein the maximum dimension between opposite surfaces of ribbon 14 is within the range of one thirty-second inch to one-quarter inch. Such range assures that the structure will have the requisite rigidity and have the proper cell size for the desired radio frequency selectivity. The upper edges 18 of ribbons 14 are shaped to form a contour that defines a concave surface 20; the surface is preferably a paraboloid to obtain optimum focusing but may take other concave shape as well. A reference point 22 in FIG. 1 indicates the position of the focus of the paraboloid of curve surface 20. A transmitting or receiving antenna is placed at focus 22 in accordance with well-known procedures.

Core 12 is supported upon a mast 24 that is attached to the reflector remote from surface 20 of the core. A suitable antenna feed (not shown) can be mounted in a passage 26 formed axially with the major axis of the paraboloid surface.

In the embodiment depicted in FIG. 2, ribbons 14 are formed of a conductive material such as aluminum and are welded or brazed by suitable material at their bond points 16. The weld points at bond 16 electrically interconnect all areas of surface 20. Thus, the reflector, if

constructed of one thirty-second inch honeycomb, will be substantially opaque for frequencies up to about cycles per second (3x10 m. wavelength) and will reflect to focus 22 all signals having frequencies below that frequency. If one-quarter inch honeycomb is used, the paraboloid surface will appear substantially opaque to frequencies up to about 10 cycles per second (2 /2 X 10- m.). Energy having wavelengths shorter than those referred to above will pass through cells 17 because the maximum dimension between opposing ribbons of a given cell exceeds one-quarter wavelength. Because all ribbons 14 are electrically bonded at 16, electrical continuity will exist over the entire paraboloid surface, and the entire surface will contribute to reflection of energy having relatively longer wavelengths.

Referring now to FIG. 3, core 12 is shown in greatly enlarged scale to emphasize an alternate embodiment of the invention. The edges 18 of the core 12 are formed with a suitable concave shape, such as paraboloid as in FIG. 1. To edges 18 is afiixed a conductive layer 28 by means of an adhesive layer 30. In the present embodiment it is not necessary for ribbons 14 or bond points 16 between adjacent ribbons to be electrically conductive, because conductive sheet 28 provides electrical interconnection between all areas of surface 20.

After a conductive sheet 28 is aflixed to core 12, concavities or dimples 32 are formed. I prefer to form the concavities by evacuating the air from each cell 17 so that atmospheric pressure acts on the top of sheet 28 to cause partial attenuation or distortion thereof into the cells. In one reflector made according to my invention sheet 28 was formed with a layer of 3 mil aluminum (.003 inch); a pressure differential across sheet 28 of approximately 2-3 atmospheres suitably formed concavities 32. Any permanently extensile material can be used to form the concavities. The pressure differential formed by evacuating cells 17 must be suflicient to exceed the elastic limit of thin sheet of permanently extensile material but in suflicient to rupture the thin sheet.

Each concavity 32 acts as a parabolic reflector for wavelengths that are short relative to the distance between opposite points on the perimeter of each concavity. Because the perimeter is determined by the ribbon edges 18, the frequency characteristics of the present embodiment are similar to those of the embodiment first described above.

The embodiments shown in FIGS. 1-3 have ribbons 14 parallel with the principal axis of the paraboloid of surface 20. Parallelism is not required; it is more important that the dimension between cell walls along surface 20 be chosen relative to the wavelength of reflected energy and the interfering energy present to secure proper electrical characteristics. The openings or depressions presented to the electro-magnetic radiation are thereby permitted to serve the intended function.

An alternate method for forming depressions 32 in sheet 28 is to affix the skin to core 12 with adhesive layer 30, and then to move a relatively hard rubber roller over the surface. If the rubber of the roller is of sufficient hardless to attenuate thin conductive sheet 28, the desired dimples or concavities 32 will be formed.

A skin 34 can be attached to ribbon edges 18 which has a plurality of randomly positioned holes 36 therethrough. (FIGS. 4 and 5). Holes 36 have a diameter no greater than the quarter wavelengths of the reflected signal, and are sufficiently numerous to pass shorter wavelength energy. Although removal of at least 80% of skin 34 by formation of holes 36 therein affords adequate conveyance of shorter wavelength energy through the skin, up to 90% can be removed without excessively weakening the structure. Unwanted energy having a quarter wavelength less than the diameter of holes 36 will pass through the holes to cells 17, and will not interfere with the energy that is to be reflected by skin 34.

A conductive layer can be formed on the concave sur- 7 face 20 of honeycomb core 12 by a conductive mesh 38 (FIG. 6). The mesh has plural individual conductors 38 that can be formed by expanded or stamped metals or by intersecting wires. The conductors are both electrically and mechanically bonded to one another at their intersecting points 40, so that electrical interconnection is attained between all areas of the surface. Mechanical bonding of mesh 38 to ribbon ends 18 of core 12 provides structural rigidity to the electrically continuous surface, so that waves reflected therefrom are focused at point 22. The openings between the conductors of mesh 38 are formed no larger than one-quarter wave length of the energy to be reflected by the device. Energy of shorter wave lengths passes through the mesh and the honeycomb core and is not reflected to an antenna. Because mesh 38 forms the conductive surface area, honeycomb core 12 in the embodiment of FIGS. 6 and 7 need not be conductive material.

It is to be noted in connection with the embodiment of FIGS. 1-2 and 4-7 that the cell forming side surfaces of ribbons 14 provide reflecting surfaces for energy entering cells 17. Such energy is reflected back and forth between opposite cell walls until it is expelled from the rear open end of cells 17 Consequently, the shorter wavelength energy that enters the cell will not interfere with the signal reflected to or from an antenna by surface 20.

Thus, I have provided a paraboloid reflector that is rigid, inexpensive to produce and frequency selective. In addition, the structure has extremely light weight. The novel method for manufacturing reflector can be used to form reflectors of different shapes; and of different frequency discriminating characteristics.

While several embodiments of my invention have been shown and described, it will be apparent that other adaptations and modifications may be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. A reflector for radio waves of the type having a concave surface for reflectively focusing electromagnetic radiation to or from an antenna spaced from. the surface comprising a body formed of cellular honeycomb material, said cellular material being formed by a plurality of ribbons bonded to one another at recurrent intervals to define plural cells, said body being arranged with edges of said ribbons shaped to define the contour of the concave surface, the maximum cross-sectional dimension of each cell bein no greater than approximately one-quarter wavelength of the radiation being reflected, and a face skin of conductive material affixed to ribbon edges of the honeycomb body forming the concave surface, said skin being dimpled to form a plurality of smaller concave surfaces in the first named surface, said smaller concave surfaces having a maximum dimension generally along the first named surface no greater than approximately one-quarter wavelength of the radiation being reflected.

2. The invention of claim 1 and wherein the perimeter of each said smaller concave surface is defined by cell forming ribbon edges.

3. A parabolic reflector for electromagnetic waves of the type having a paraboloid surface for reflectively focusing radio frequency energy to or from an antenna disposed substantially at the focus of the paraboloid comprising a body formed of cellular honeycomb material, said honeycomb material being formed by flat ribbons bonded together at recurrent intervals to form cells, said ribbons being disposed generally parallel with the principal axis of the paraboloid, said cellular material being formed with the maximum distance between opposite ribbons of a cell no greater than approximately one-quarter of the wavelength being reflected, and a face skin of conductive material aflixed to ribbon edges of the honeycomb body forming the paraboloid, said skin being dimpled to form a plurality of small concave surfaces in the paraboloid surface, said small concave surfaces having a maximum dimension generally along the paraboloid surface no References Cited UNITED STATES- PATENTS Titus 29-471 Dunkle et al 156-197 Mathis 343-912 Williams 343-912 Hansell 343-840 10 Giuliani 343-912 Crandon 343-912 6 Svensson et a1 343-838 Swallow et a1 343-915 Eisentraut 343-914 Dunkle et a1. 343-912 Mathis 343-915 Great Britain.

US. Cl. X.R. 

1. A REFLECTOR FOR RADIO WAVES OF THE TYPE HAVING A CONCAVE SURFACE FOR REFLECTIVELY FOCUSING ELECTROMAGNETIC RADIATION TO OR FROM AN ANTENNA SPACED FROM THE SURFACE COMPRISING A BODY FORMED OF CELLULAR HONEYCOMB MATERIAL, SAID CELLULAR MATERIAL BEING FORMED BY A PLURALITY OF RIBBONS BONDED TO ONE ANOTHER AT RECURRENT INTERVALS TO DEFINE PLURAL CELLS, SAID BODY BEING ARRANGED WITH EDGES OF SAID RIBBONS SHAPED TO DEFINE THE CONTOUR OF THE CONCAVE SURFACE, THE MAXIMUM CROSS-SECTIONAL DIMENSION OF EACH CELL BEIN NO GREATER THAN APPROXIMATELY ONE-QUARTER WAVELENGTH OF THE RADIATION BEING REFLECTED, AND A FACE SKIN OF CONDUCTIVE MATERIAL AFFIXED TO RIBBON EDGES OF THE HONEYCOMB BODY FORMING THE CONCAVE SURFACE, SAID SKIN BEING DIMPLED TO FORM A PLURALITY OF SMALLER CONCAVE SURFACES IN THE FIRST NAMED SURFACE, SAID SMALLER CONCAVE SURFACES HAVING A MAXIMUM DIMENSION GANERALLY ALONG THE FIRST NAMED SURFACE NO GREATER THAN APPROXIMATELY ONE-QUARTER WAVELENGTH OF THE RADIATION BEING REFLECTED. 